prog: fix encoding for exec of result args

ResultArg might have const value.

Also add a test.

2 files changed, 20 insertions, 5 deletions

diff --git a/prog/encodingexec.go b/prog/encodingexec.go
index ba2efcd37..403b09f88 100644
--- a/prog/encodingexec.go
+++ b/prog/encodingexec.go
@@ -253,11 +253,19 @@ func (w *execContext) writeArg(arg Arg, pid int, csumMap map[Arg]CsumInfo) {
 		w.write(a.Type().BitfieldOffset())
 		w.write(a.Type().BitfieldLength())
 	case *ResultArg:
-		w.write(ExecArgResult)
-		w.write(a.Size())
-		w.write(w.args[a.Res].Idx)
-		w.write(a.OpDiv)
-		w.write(a.OpAdd)
+		if a.Res == nil {
+			w.write(ExecArgConst)
+			w.write(a.Size())
+			w.write(a.Val)
+			w.write(0) // bit field offset
+			w.write(0) // bit field length
+		} else {
+			w.write(ExecArgResult)
+			w.write(a.Size())
+			w.write(w.args[a.Res].Idx)
+			w.write(a.OpDiv)
+			w.write(a.OpAdd)
+		}
 	case *PointerArg:
 		w.write(ExecArgConst)
 		w.write(a.Size())
diff --git a/prog/encodingexec_test.go b/prog/encodingexec_test.go
index b7a6b9463..2d96a47e0 100644
--- a/prog/encodingexec_test.go
+++ b/prog/encodingexec_test.go
@@ -250,6 +250,13 @@ func TestSerializeForExec(t *testing.T) {
 				instrEOF,
 			},
 		},
+		{
+			"syz_test$res1(0xffff)",
+			[]uint64{
+				callID("syz_test$res1"), 1, argConst, 4, 0xffff, 0, 0,
+				instrEOF,
+			},
+		},
 	}
 
 	buf := make([]byte, ExecBufferSize)